This invention relates generally to ultrasonic transducers and more particularly to a laser driven, interferometric optical transducer for measuring ultrasonic vibration on an object surface.
The measurement of material surface discontinuities has traditionally been accomplished by using piezoelectric transducers to measure ultrasonic surface vibration induced in the material. However, in many applications the wavelength of the ultrasound is substantially smaller than the diameter of the transducer. For example, when the ultrasound frequency is 10 MHz, its wavelength is about 0.5 mm, while the typical transducer may be 12 mm in diameter. This size differential makes the transducer very directional.
Piezoelectric transducers, even if small enough to overcome the directionality problem, are contact coupled to the surface being measured. Often, an impedance matching material is interposed between the transducer and the surface being measured in order to obtain proper impedance matching.
Furthermore, piezoelectric transducers are not easily calibrated. Usually some secondary device must be employed in order to obtain proper calibration.
One solution to these problems is to use an optical transducer. Such transducers can have very small diameters, and since they are essentially massless, they have very small impedance. A laser beam properly focused on a surface may be used as an ultrasonic detector. An additional advantage of a laser ultrasonic detector is that it can be scanned across a surface by simply moving a mirror.